LM3405 : Constant Current Regulator for Powering LEDs
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The document provides an overview of the LM3405 constant current regulator for powering LEDs. It describes the key features of the LM3405 such as its operating voltage range, package type, switching frequency, and enable/dimming functionality. It also includes diagrams of the internal block diagram and typical application circuit as well as guidance on designing the inductor, capacitors, and diodes in the circuit.
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LM3405 : Constant Current Regulator for Powering LEDs
- 7. Methods of Deriving V Boost FIG2. V BOOST derived from V IN through a series Zener FIG 1. V BOOST derived from V OUT FIG3. V BOOST derived from V IN through a Shunt Zener
- 9. PWM Dimming
- 10. Driving Multiple LED Strings
Editor's Notes
- Welcome to the training module on National Semiconductor LM3405 Constant Current Regulator for Powering LEDs . The intent of this training module is to introduce the operation of LM3405.
- Integrated with a 1A power switch, the LM3405 is a current mode control switching buck regulator designed to provide a simple, high efficiency solution for driving high power LEDs. With a 0.205V reference voltage feedback control to minimize power dissipation . Switching frequency is internally set to 1.6MHz, allowing small surface mount inductors and capacitors to be used. The LM3405 utilizes current-mode control and internal compensation offering ease of use and predictable, high performance regulation over a wide range of operating conditions. Additional features include user accessible EN/DIM pin for enabling and PWM dimming of LEDs, thermal shutdown, cycle-by-cycle current limit and over-current protection.
- The LM3405 is a PWM, current-mode control switching buck regulator designed to provide a simple, high efficiency solution for driving LEDs with a preset switching frequency of 1.6MHz. This high frequency allows the LM3405 to operate with small surface mount capacitors and inductors, resulting in LED drivers that need only a minimum amount of board space. The LM3405 is internally compensated, simple to use, and requires few external components. The LM3405 supplies a regulated output current by switching the internal NMOS power switch at constant frequency and variable duty cycle. Capacitor C3 and diode D2 in Figure 1 are used to generate a voltage V BOOST . The voltage across C 3 , V BOOST - V SW , is the gate drive voltage to the internal NMOS power switch.
- A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS power switch. During this on-time, the SW pin voltage (V SW ) swings up to approximately V IN , and the inductor current (I L ) increases with a linear slope. IL is measured by the current sense amplifier, which generates an output proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and V REF . When the PWM comparator output goes high, the internal power switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through the catch diode D1, which forces the SW pin to swing below ground by the forward voltage (V D1 ) of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output current (I F ) through the LED, by forcing FB pin voltage to be equal to V REF (0.205V).
- To properly drive the internal NMOS switch during its on-time, V BOOST needs to be at least 2.5V greater than V SW . Large value of V BOOST - V SW is recommended to achieve better efficiency by minimizing both the internal switch ON resistance (R DS(ON) ), and the switch rise and fall times. However, V BOOST - V SW should not exceed the maximum operating limit of 5.5V. When the LM3405 starts up, internal circuitry from VIN supplies a 20mA current to the BOOST pin, flowing out of the BOOST pin into C3. This current charges C3 to a voltage sufficient to turn the switch on. The BOOST pin will continue to source current to C3 until the voltage at the feedback pin is greater than 123mV.
- There are 4 methods to derive V BOOST. The first method is shown in the Internal Block Diagram. Capacitor C3 is charged via diode D2 by V IN . The second method for deriving the boost voltage is to connect D2 to the output as shown in Figure 1. The gate drive voltage in this configuration is: V BOOST - V SW = V OUT – V D2 + V D1 Since the gate drive voltage needs to be in the range of 2.5V to 5.5V, the output voltage V OUT should be limited to a certain range. The third method, shown in Figure 2, can be used in the applications where both V IN and V OUT are greater than 5.5V. In these cases, C3 cannot be charged directly from these voltages; instead C3 can be charged from V IN or V OUT minus a zener voltage (V D3 ) by placing a zener diode D3 in series with D2 as shown When using a series zener diode from the input, the gate drive voltage is V IN - VD3 - V D2 + V D1 . The fourth method, shown in Figure 3 can be used in an application which has an external low voltage rail, V EXT . C3 can be charged through D2 from V EXT , independent of V IN and V OUT voltage levels. Again for best performance, ensure that the gate drive voltage, V EXT - V D2 + V D1 , falls in the range of 2.5V to 5.5V.
- Here shows the typical application circuit using the LM3405 regulator driving a single LED. The boost voltage can be derived from V IN or V OUT . The BOOST to SW voltage (V BOOST -V SW ) should never exceed the operating limit of 5.5V, and must be greater than 2.5V for optimum efficiency. In this circuit, the maximum V IN is 5V and therefore the boost voltage is derived from V IN . The LM3405 driver tightly regulates the FB-pin voltage to 205 mV (typical) and this allows the LED current (IF) to be set by the listed equation. In case of an over current, the internal current limit will trigger and turn off the internal power switch on a cycle-by-cycle basis. In case of an over voltage (sensed at the FB pin), the internal power switch is turned off.
- The LEDs can be dimmed by applying a PWM-logic signal to the EN/DIM pin of the LM3405 driver as shown in the figure. A logic high at V_PWM enables the IC, and a logic low disables the IC. In this manner, the LED current is turned on and off. In order to eliminate flicker, the lowest PWM-dimming frequency is normally chosen to be above 100 Hz. The upper end of the PWM frequency is determined by the turn-on delay of the LM3405. The average LED current is controlled by TON, TPER, or both and is perceived by the eye as a brightness change.
- The LM3405 LED driver can be used to drive multiple LED strings in parallel as shown in the Figure. The current in the primary branch (with LED1 and LED2) will be tightly regulated by the feedback loop. The current in the secondary branch (with LED3) will be regulated based on VOUT, forward voltage of LED3, and R2. The value of R2 can be adjusted to get the desired brightness for LED3.
- Multiple LEDs can be paralleled and connected between V OUT and FB as shown in the figure. The voltage at the FB pin is regulated to 205 mV by the control loop and therefore the current in R1 is fixed. V OUT is determined by the LED with the highest forward voltage. This solution has the advantage of having a single resistor set the total currents in the LEDs but has no control of equal current sharing between the LEDs.
- The inductor value determines the output ripple current Lower inductor values decrease the size of the inductor, but increases the output ripple current. An increase in the inductor value will decrease the output ripple current. The ratio of ripple current to LED current is optimized when it is set between 0.3 and 0.4 at 1A LED current. The table lists the recommended inductance range.
- An input capacitor is necessary to ensure that VIN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, voltage rating, RMS current rating, and ESL (Equivalent Series Inductance). A 10μF multilayer ceramic capacitor (MLCC) is a good choice for most applications. The output capacitor is selected based upon the desired reduction in LED current ripple. A 1μF ceramic capacitor results in very low LED current ripple for most applications. Due to the high switching frequency, the 1μF capacitor alone (without feed-forward capacitor C4) can filter more than 90% of the inductor current ripple for most applications where the sum of LED dynamic resistance and R1 is larger than 1Ω. The feed-forward capacitor (C4) connected in parallel with the LED string is required to provide multiple benefits to the LED driver design. For most applications, a 1μF ceramic capacitor is sufficient. In fact, the combination of a 1μF feed-forward ceramic capacitor and a 1μF output ceramic capacitor leads to less than 1% current ripple flowing through the LED.
- The catch diode (D1) conducts during the switch off-time. A Schottky diode is required for its fast switching time and low forward voltage drop. The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin. To improve efficiency, choose a Schottky diode with a low forward voltage drop. A standard diode such as the 1N4148 type is recommended. For VBOOST circuits derived from voltages less than 3.3V, a small-signal Schottky diode is recommended for better efficiency. A good choice is the BAT54 small signal diode.
- Thank you for taking the time to view this presentation on LM3405 LED Driver. If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the National Semiconductor site – link shown – or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.